(Not Applicable)
(Not Applicable)
The present invention relates generally to chip stacks, and more particularly to a chip stack including a uniquely configured flex circuit which allows different types of packaged chips to be placed into electrical communication with each other in a stacked configuration.
Multiple techniques are currently employed in the prior art to increase memory capacity on a printed circuit board. Such techniques include the use of larger memory chips, if available, and increasing the size of the circuit board for purposes of allowing the same to accommodate more memory devices or chips. In another technique, vertical plug-in boards are used to increase the height of the circuit board to allow the same to accommodate additional memory devices or chips.
Perhaps one of the most commonly used techniques to increase memory capacity is the stacking of memory devices into a vertical chip stack, sometimes referred to as 3D packaging or Z-Stacking. In the Z-Stacking process, from two (2) to as many as eight (8) memory devices or other integrated circuit (IC) chips are interconnected in a single component (i.e., chip stack) which is mountable to the xe2x80x9cfootprintxe2x80x9d typically used for a single package device such as a packaged chip. The Z-Stacking process has been found to be volumetrically efficient, with packaged chips in TSOP (thin small outline package) or LCC (leadless chip carrier) form generally being considered to be the easiest to use in relation thereto. Though bare dies or chips may also be used in the Z-Stacking process, such use tends to make the stacking process more complex and not well suited to automation.
In the Z-Stacking process, the IC chips or packaged chips must, in addition to being formed into a stack, be electrically interconnected to each other in a desired manner. There is known in the prior art various different arrangements and techniques for electrically interconnecting the IC chips or packaged chips within a stack. Examples of such arrangements and techniques are disclosed in Applicant""s U.S. Pat. No. 4,956,694 entitled INTEGRATED CIRCUIT CHIP STACKING issued Sep. 11, 1990, U.S. Pat. No. 5,612,570 entitled CHIP STACK AND METHOD OF MAKING SAME issued Mar. 18, 1997, and U.S. Pat. No. 5,869,353 entitled MODULAR PANEL STACKING PROCESS issued Feb. 9, 1999.
The various arrangements and techniques described in these issued patents and other currently pending patent applications of Applicant have been found to provide chip stacks which are relatively easy and inexpensive to manufacture, and are well suited for use in a multitude of differing applications. However, one major drawback associated with currently known chip stack arrangements and chip stacking techniques is the inability to quickly, easily and inexpensively create chip stacks including dissimilar packaged chips, i.e., packaged chips of differing types.
The present invention provides yet a further alternative arrangement and technique for forming a chip stack which involves the use of a uniquely configured flex circuit or substrate specifically adapted to allow multiple chip packages including different types of packaged chips to be electrically interconnected to each other in a stacked configuration or arrangement. For example, a chip stack constructed in accordance with the present invention incorporating the flex circuit thereof may include a mixture of TSOP and BGA (ball grid array) packaged chips or devices, or a mixture of discrete or passive encased devices. The flex circuit also provides various advantages in the assembly of the chip stack, including significantly greater ease in achieving and maintaining the alignment between the chip packages having the dissimilar packaged chips. These, and other advantages of the present invention, will be discussed in more detail below.
In accordance with the present invention, there is provided a chip stack which comprises a flex circuit or similar substrate. The flex circuit itself comprises a flexible substrate having opposed, generally planar top and bottom surfaces. Disposed on the top surface of the substrate in spaced relation to each other are first, second, and third top conductive patterns. Similarly, disposed on the bottom surface of the substrate in spaced relation to each other are first, second and third bottom conductive patterns. The first top and bottom conductive patterns are electrically connected to each other, as are the second top and bottom conductive patterns and the third top and bottom conductive patterns.
In addition to the flex circuit, the chip stack comprises at least first and second identically configured top chip packages. The first top chip package is electrically connected to the first top conductive pattern, with the second top chip package being electrically connected to the first top chip package. In addition to the top chip packages, the chip stack includes at least first and second identically configured bottom chip packages. The first bottom chip package is electrically connected to the second bottom conductive pattern, with the second bottom chip package being electrically connected to the third bottom conductive pattern. In the present chip stack, the substrate is folded such that the second top conductive pattern is electrically connected to the second top chip package, and the third top conductive pattern is electrically connected to the first bottom chip package.
In the present chip stack, each of the first and second top chip packages include a first packaged chip, with each of the first and second bottom chip packages including a second packaged chip differing from the first packaged chip. The first packaged chip of each of the first and second top chip packages is preferably a TSOP device, with the second packaged chip of each of the first and second bottom chip packages preferably being a BGA device.
In addition to the first and second packaged chips, the first and second top chip packages and the first and second bottom chip packages each further comprise a frame having a conductive pad array disposed thereon. In the present chip stack, the conductive pad array of the frame of the first top chip package is electrically connected to the first top conductive pattern, with the conductive pad array of the second top chip package being electrically connected to the conductive pad array of the first top chip package. Additionally, the conductive pad array of the first bottom chip package is electrically connected to the second bottom conductive pattern, with the conductive pad array of the second bottom chip package being electrically connected to the third bottom conductive pattern. The first packaged chip of each of the first second top chip packages comprises a body defining an opposed pair of sides and having a multiplicity of conductive leads protruding from each of the sides thereof. The conductive leads of the first packaged chip of each of the first and second top chip packages is electrically connected to a respective one of the conductive pad arrays thereof. The second packaged chip of each of the first and second bottom chip packages itself comprises a body having opposed, generally planar top and bottom surfaces and a multiplicity of conductive contacts protruding from the bottom surface. The conductive contacts of the second packaged chip of each of the first and second bottom chip packages are electrically connected to a respective one of the conductive pad arrays thereof via a respective one of the second and third conductive patterns.
In the present chip stack, each of the frames preferably has a generally rectangular configuration defining opposed pairs of longitudinal and lateral side segments. Similarly, the substrate preferably has a generally rectangular configuration defining opposed pairs of longitudinal and lateral peripheral edge segments. The substrate, the first and second top chip packages, and the first and second bottom chip packages are sized relative to each other such that the lateral side segments of the frames do not protrude beyond the longitudinal peripheral edge segments of the substrate in the fully assembled chip stack.